In the process for the production of .alpha.-anthraquinonesulfonic acids by sulfonating anthraquinone in solvent, a metal type catalyst is required. Different catalysts and solvents have been disclosed in the past in order to accomplish the sulfonation reaction with a high conversion ratio and .alpha.-selectivity, and methods for recovering the catalyst also have been discussed. However, a process which has a high conversion ratio, .alpha.-selectivity, and in which the catalyst recovered therefrom retains the same activity has not been disclosed heretofore.
A well known process for producing 1-anthraquinonesulfonic acid, as is taught in Kirk-Othmer, "Encyclopedia of Chemical Technology", 3rd edition (1978), vol. 2, P.729, wherein anthraquinone is sulfonated in oleum in the presence of mercury salt, requires that the sulfonation reaction is interrupted as soon as about 45% of the starting anthraquinone have been converted, since otherwise disulfonation will be predominant.
U.S. Pat. No. 3,763,191 teaches a process for producing .alpha.-anthraquinonesulfonic acids in substantially higher yield and higher selectivity, especially if palladium or compounds containing the same are used. However, the degree of sulfonation and .alpha.-selectivity shown in the examples of said U.S. patent are still less than desired and furthermore, a process for recovering the catalyst solubilized in the reaction mixture is not disclosed therein.
U.S. Pat. No. 3,792,065 discloses a process for the production of .alpha.-anthraquinonesulfonic acids which are substantially devoid of mercury, in which mercury introduced in the form of the sulfate as the catalyst is seperated from the sulfonation mixture as metal by the addition of a reducing agent.
New catalysts which promote selectively the .alpha.-sulfonation of anthraquinone in liquid sulfur dioxide as a solvent are taught addressed in, "Palladium-Catalyzed .alpha.-Monosulfonation of Anthraquinone by Sulfur Trioxide in Liquid Sulfur Dioxide" by Yasuziro Kawabata, et al., appearing in Journal of the Chemical Society of Japan, 1980, (3), p. 322-326, wherein the use of liquid sulfur dioxide and palladium containing compounds in the sulfonation reaction resulted in about 100% conversion of anthraquinone and 76% yield of 1-anthraquinonesulfonic acid with 99% .alpha.-selectivity. Further, Kawabata, et al. disclose a process of reprecipitation of the solubilized palladium on carbon by hydrogen treating, in which the pH value of the reaction mixture was adjusted to &gt;7, and then 20 atm hydrogen was introduced to deposit the solubilized Pd on carbon. However, the conversion of anthraquinone was found to have decreased to 70% as the recovered Pd-carbon was recycled. Additionally, air or oxygen was introduced into the mixture after the Pd-carbon had been removed in order to precipitate a salt of the monoanthraquinonesulfonic acid.